Abstract. Knowledge of the viscosity of particles containing secondary organic material (SOM) is useful for predicting reaction rates and diffusion in SOM particles. In this study we investigate the viscosity of SOM particles as a function of relative humidity and SOM particle mass concentration, during SOM synthesis. The SOM was generated via the ozonolysis of α-pinene at <5% relative humidity (RH). Experiments were carried out using the poke-and-flow technique, which measures the experimental flow time (τexp, flow) of SOM after poking the material with a needle. In the first set of experiments, we show that τexp, flow increased by a factor of 3600 as the RH increased from <0.5 RH to 50% RH, for SOM with a production mass concentration of 121µgm−3. Based on simulations, the viscosities of the particles were between 6 × 105 and 5 × 107Pas at <0.5% RH and between 3 × 102 and 9 × 103Pas at 50% RH. In the second set of experiments we show that under dry conditions τexp, flow decreased by a factor of 45 as the production mass concentration increased from 121 to 14000µgm−3. From simulations of the poke-and-flow experiments, the viscosity of SOM with a production mass concentration of 14000µgm−3 was determined to be between 4 × 104 and 1.5 × 106Pas compared to between 6 × 105 and 5 × 107Pas for SOM with a production mass concentration of 121µgm−3. The results can be rationalized by a dependence of the chemical composition of SOM on production conditions. These results emphasize the shifting characteristics of SOM, not just with RH and precursor type, but also with the production conditions, and suggest that production mass concentration and the RH at which the viscosity was determined should be considered both when comparing laboratory results and when extrapolating these results to the atmosphere.

The effect of several experimental parameters on the viscosity of secondary organic material (SOM) generated from the ozonolysis of α-pinene has been studied. The results demonstrate that the viscosity of SOM depends on the particle mass concentration at which SOM is produced, and the relative humidity (RH) at which the SOM is studied. Hence, particle mass concentration and RH should be considered when comparing experimental results for SOM, or extrapolating laboratory results to the atmosphere.

The effect of several experimental parameters on the viscosity of secondary organic material...